The invention relates to a vehicle transmission shift control system, and more particularly, to adaptive load-based powershift transmission shift control system which responds to various conditions, including the functional status of an auxiliary function driven by the engine.
A known production tractor, such as a John Deere 8000 series tractor, includes a powershift transmission (PST) controlled by a plurality of solenoid operated transmission clutch control valves, a power take off (PTO) controlled by a solenoid operated clutch. These and other functional elements are controlled by microprocessor-based vehicle control unit (VCU). Such a known production tractor also includes a conventional hitch operated by a solenoid operated hitch control valve and a plurality of solenoid operated selective control valves (SCV). The hitch solenoid and the SCV solenoid are controlled by a microprocessor-based hitch and SCV control unit which communicates with the vehicle control unit (VCU) by a conventional commercially available data bus. The commercially available VCU executes a shift control program which controls the shifting of the PST from one gear to another by controlling pressure applied to the transmission control clutches as a function of, among other things, the load as represented by the fuel flow or load signal from an electronic engine control unit (ECU). To achieve smooth shifts, generally, a higher clutch pressure is used under high load conditions, and a lower clutch pressure is used under lower load conditions.
However, when the engine is driving auxiliary functions, such as the PTO or one or more SCVs, the load signal from the ECU will not accurately represent the load which is actually applied to the transmission. This can adversely effect the smoothness of certain shifts. The production John Deere 8000 series tractor includes a diagnostic mode feature wherein the operator can manually adjust the load signal factor to compensate for the effects of operation of an auxiliary function. However, with this diagnostic mode feature, the compensation cannot be automatically or rapidly modified to adjust for rapidly changing circumstances, nor can the compensation be modified during an actual shift of the transmission.